In many industrial textile applications, especially in knitting machines, it is often necessary to keep yarns which are to be furnished to knitting stations or other locations at a constant tension. This is especially important in flatbed knitting machines, which because of the reciprocating motion of the yarn guide (carriage) have a yarn consumption that fluctuates very greatly over time. A corresponding yarn feeder must then furnish the yarn at a speed that repeatedly varies abruptly over time. If the yarn tension changes, for instance during, before or after the reversal of motion of the yarn guide, then the mesh size of the knitted product changes, which impairs its appearance, elasticity, and quality. In this respect, the edge regions of knitted goods made on flatbed knitting machines are especially critical.
Special demands must be made of the constancy of tension when elastic yarns (e.g. Spandex.TM.) are supplied, which are for instance knitted jointly with other yarns. To keep the yarn tension constant, it is necessary to monitor the tension constantly and to regulate the yarn feed quantity accordingly.
To that end, a yarn feeder for elastic yarns is known, for instance from German Patent Disclosure DE 195 37 215 A1, that is intended for use in flatbed knitting machines. The yarn feeder is used to feed Spandex.TM. yarns and has a yarn feed wheel driven by an electric motor. The electric motor is triggered by a closed control loop that detects the current yarn tension with a yarn tension sensor. The yarn tension sensor has a peg that can be deflected crosswise to the yarn travel direction, and the yarn is guided over this peg at an obtuse angle. The peg deflection corresponds to the yarn tension and is detected by a suitable travel sensor.
A yarn feeder for knitting machines is also known from U.S. Pat. No. 3,858,416; it likewise has a yarn feed wheel which is driven by a motor. The motor is triggered by a closed control loop that detects the yarn tension with a yarn tension sensor. The yarn tension sensor has a deflectable peg over which the yarn travels.
From German Patent Disclosure DE 39 42 341 A1, a force sensor for monitoring yarn tensions is known in which a sensor element is supported on a spring parallelogram. The deflection of the sensor element is transmitted to a bending body that is provided with variable resistance, so that the deflection of the sensor element and thus the yarn tension can be detected electrically.
The constancy of tension is of major importance especially when elastic yarns for making elastic knitted goods are being supplied. Even minimal fluctuations, and especially longer-lasting changes, lead to changes or variations in quality. It is therefore important that the yarn tension be kept stable over long periods of time, that is, over the course of hours, days and months.
Knitting machines and yarn feeders are often used in large factory spaces in which the temperature varies, both over the course of the day and depending on how long the machines have been running, and not least because of the heat loss from the knitting machines. Thus the temperatures of the yarn tension sensors vary as well, which despite temperature compensation means that may be present can have an effect on their output signal. Persistent dirt deposits can also lead to a change in the sensor output signal, for instance if deposits on a peg for detecting the yarn tension increase the total weight of the peg and thus shift the zero point of the signal.